This invention relates to currency paper security threads, and more particularly to a capacitance-based verification device for a security thread embedded within currency paper.
It is known in the art of currency and banknote papers to incorporate a security thread therein. The thread may comprise a plastic film having aluminum characters formed thereon. The thread is embedded within the currency and is not present on the currency surface. Such security threads are described in greater detail in Timothy Crane U.S. Pat. Nos. 4,652,015 and 4,761,205, both of which are assigned to Crane & Co., Inc.
The security thread described in these patents has printed characters thereon of extreme fine-line clarity and high opacity such that human readability of the printing is possible by means of transmitted light. Yet the printing remains completely indiscernible under reflected light. To check the authenticity of the thread, the currency is normally placed under an intense light source and the characters observed by the human eye. However, in commercial situations where such an intense light source is unavailable, thus making a human check for thread presence and authenticity virtually impossible, it is desirable to provide means for automatically determining the thread's presence and authenticity.
Various means are known for providing verification of the presence and authenticity of the security thread within the currency. For example, Crane et al. U.S. Pat. No. 4,980,569 ('569) discloses a verification device comprising two optical light source/detector pairs disposed on opposite sides of a proffered currency. The source and detector pairs are arranged for transmission and reception of optical energy through the currency if the thread is not present. Also, the source and detector pairs can determine the presence of a counterfeit thread on the currency surface by checking for light reflected off the currency surface. Thus, the '569 patent provides a two-fold test wherein the thread, to be genuine, must be detected under transmitted light and not be detected under reflected light. However, the device in the '569 patent may give a false indication of the authenticity of a counterfeit currency when a pencil line is drawn on the currency surface at the normal thread location.
In an attempt to overcome the shortcomings of the '569 patent, Crane et al. U.S. Pat. No. 5,151,607, discloses a verification device comprising the optical means of the '569 patent in combination with a magnetic detector, magnetic reader or non-ferrous-metal detector. The latter detectors determine the presence of the security thread, while the optical means determines whether the thread is properly within the currency or improperly disposed on either surface.
Accordingly, it is a primary object of the present invention to provide an improved security thread detector.
It is a general object to provide a detector that can determine the authenticity of currency at a rapid rate.
It is yet another object to provide a detector that can verify the presence of a security thread embedded within currency paper through use of a capacitance bridge.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.